In the field of magnetic hard disk recording, it is important that the magnetic reading heads, or sliders, be located at a predetermined, known, relatively small spacing from the rotating magnetic disk upon which the digital information is stored. In practice, after the disk is rotating at normal relatively high speed, the magnetic heads are advanced and are allowed to seek their natural dynamic "flying" height above the disk, as a result of the balance between (1) a spring biasing force urging the slider toward the disk, and (2) air pressure resulting from the high speed rotation of the disk relative to the slider which tends to push the slider away from the disk. The "flying height" of the slider has been reduced in modern hard disk memories, from more than 1000 nanometers, (in the order of 40 micro-inches) down to close to 100 nanometers (in the order of 4 micro-inches).
It has been proposed to estimate flying heights of sliders by using a glass or quartz disk substituted for the hard magnetic disk, by directing illumination substantially normal to the disk to the interface between the transparent disk and the slider, and observing the interference fringes and color changes using Newton's color scale (as set forth in Newton's "OPTICKS") observed in the combined reflections from the two adjacent surfaces. However, such techniques require highly skilled personnel and are relatively inaccurate.
Accordingly, a principal object of the present invention is to provide a substantially automatic method and apparatus for measuring spacings extending to substantially less than 1000 nanometers with one nanometer being equal to 10.sup.-9 meters.